Continuous preparation of heat-vulcanizable silicone compositions

ABSTRACT

High levels of treated fumed silica, processing fluid and high molecular weight silicone polymer are continuously compounded into a homogeneous silica filled heat-vulcanizable silicone composition by forming a premix in a continuous annular layer mixer and continuously discharging the premix into a compounding apparatus for compounding to form the filled heat-vulcanizable silicone composition.

BACKGROUND OF THE INVENTION

The invention relates to a process for continuously preparingheat-vulcanizable silicone compositions.

A heat-vulcanizable silicone composition comprises a high viscositysilicone polymer, an inorganic reinforcing filler and various additivesthat aid processing or impart desired final properties to thecomposition. A vulcanizing agent can be added and the compositionheat-cured to fabricate silicone rubber moldings such as gaskets,medical tubing and computer keypads.

Typically, the heat-vulcanizable silicone composition is produced bykneading a high-viscosity polydiorganosiloxane, the inorganic filler andadditives by means of a batch kneading machine such as a high intensityBanbury mixer or a low intensity double arm dough mixer. In thisprocess, polydiorganosiloxane, inorganic filler, treating agents andadditives are batch mixed until desired properties are obtained. InKasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate ofpolydiorganosiloxane, inorganic filler and treating agents is formed bya high speed mechanical shearing mixer. The resulting premix is furthercompounded in a same-direction double screw extruder. The premix isformed in a first step wherein a diorganopolysiloxane having a viscosityat 25° C. of 1×10⁵cP or more, an inorganic filler and a treating agentare mixed in a high speed mechanical shearing machine to provide aflowable particulate mixture in which each ingredient is present in asubstantially uniform, finely dispersed state. The flowable particulatemixture is then fed at a constant feed rate into a kneading andextruding machine that has two screws rotating in the same direction.

A batch process requires long mixing times and large amounts of energy.Non-homogeneous shear and extensional stress across a commercial sizedbatch can result in non-uniform size distribution of filler that resultsin variations in properties. Batches processed at different times may becharacterized by different physical properties. The batch process islabor, energy and capital intensive and produces materials of onlymarginal consistency.

In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate ofpolydiorganosiloxane, inorganic filler and treating agents is formed ata temperature in the range of about 200° C. to 300° C. in a co-rotatingcontinuous double screw extruder. The preconcentrate is then compoundedand heat treated at 150° C. to 300° C. in a counter-rotating, doublescrew extruder. However, a process that requires two extruders isexpensive and requires significant processing area.

However with these processes, throughput is limited. There is a need fora process that provides improved throughput and which can be adapted asa low cost process that can efficiently utilize a single extruder tocontinuously and consistently produce a full range of both low viscosityand high viscosity silicone elastomers from filler, additive andpolymer.

SUMMARY OF THE INVENTION

The invention provides a process that compounds high levels of filler,processing fluid and silicone polymer into homogeneous filledheat-vulcanizable silicone compositions with requisite reinforcingproperties and levels of volatiles. The process comprises continuouslyfeeding filler and silicone polymer to a high speed mixing stage to forma free-flowing particulate concentrate. The concentrate is continuouslydischarged from the mixing stage to a compounding apparatus for furtherprocessing.

In another aspect, the invention relates to a process of forming apremix of filler and silicone polymer. In the process, a filler is mixedwith a silicone polymer in a continuous annular layer mixer and a filledsilicone polymer premix is discharged from the mixer.

In another aspect, the invention relates to a process of compounding afilled heat-vulcanizable silicone composition wherein a filler is mixedwith a silicone polymer in a high speed continuous mixer at an elementtip speed of between about 3 m/s and about 100 m/s to form a premix. Thepremix is then discharged to a next processing apparatus.

In another aspect, the invention relates to a continuous annular layermixer having a sequence of sections comprising at least a first sectioncomprising a forward pitched mixing element, a second section comprisinga neutral or forward pitched cutting element and a third sectioncomprising a rearward pitched mixing element.

In yet still another aspect, the invention relates to a compoundingapparatus, comprising a first stage continuous annular layer mixer andat least one subsequent stage comprising an extruder connected to thefirst stage to permit continuous discharge of processed material fromthe first stage to the second stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a continuous heat-vulcanizablesilicone composition compounding process and apparatus;

FIG. 2 is a schematic representation of a continuous heat-vulcanizablesilicone composition compounding process and apparatus;

FIG. 3 is a side elevation view of a continuous annular layer mixer;

FIGS. 4 and 5 are perspective views of mixer elements;

FIG. 6 is a schematic representation of elements illustrating elementpitch;

FIG. 7 is a schematic representation of a view along the shaftlongitudinal axis of a continuous annular layer mixer;

FIG. 8 is a schematic representation of another view along the shaftlongitudinal axis of a continuous annular layer mixer; and

FIG. 9 is a reference compass for determining element pitch.

DETAILED DESCRIPTION OF THE INVENTION

Banbury or dough mixers are known for the batch compounding of fillerswith silicone polymers. The compounding operation has two distinctsteps; the first step involves the wetting of the filler by the polymer,while the second step involves breaking down of agglomerates and uniformdispersing of filler in polymer. Sufficient dispersion of filler inpolymer is important. Any large undispersed agglomerates result in poormechanical properties since they can act as failure initiating flaws.

In batch or continuous processes, a treating agent can be dispersedalong with the silicone polymer either with or before the addition offiller. In these processes, large interfacial forces develop betweensilicone polymer and free, unreacted silanol groups present in filler.The treating agent must diffuse through the bulk of the high molecularweight silicone polymer and penetrate a rigid silicone polymer/fillerinterface to reach the filler for treatment. Mixing intensity must beincreased to overcome the large interfacial forces and to permitpenetration of the treating agent to access the silanol groups. Anincrease in mixing intensity causes an undesirable increase in materialtemperature.

According to the invention, a free-flowing particulate concentrate ofsilicone and filler is continuously formed in a high feed mixing stage.The concentrate is continuously discharged from the mixing stage to acompounding apparatus for further processing. The mixing stage cancomprise a continuous annular layer mixer. A continuous annular layermixer comprises a cylindrical mixing trough wherein material to be mixedis propelled along a helical path along the axis of the trough in theform of a ring contiguous with the cylindrical mixer wall. A typicalcontinuous annular layer mixer is disclosed in Erich et al., U.S. Pat.No. 5,018,673 describing a mixer comprising an essentially horizontallyarranged cylindrical housing, which is provided at a first end with amaterial supply pipe for a continuous supply of material and, at asecond end, with a material discharge pipe for a continuous removal ofmaterial. The cylindrical housing encloses a mixing apparatus, which isarranged coaxially in the housing. The mixing apparatus is driveable athigh speeds. The apparatus comprises mixing tools, which projectessentially radially from the apparatus into the vicinity of the housinginner wall. The mixer includes a draw-in zone that is associated with amaterial supply pipe and a wetting zone that is provided downstream inan axial conveying direction of the draw-in zone. The mixer alsoincludes means provided in the wetting zone for the admission of liquidinto a form of a ring of material. The ring is helically conveyed andmoved through the mixer on the housing inner wall. The mixer furtherincludes means for the separation of clusters. The means includes aplurality of cutting devices provided in a radial plane relative to theshaft of the mixing apparatus and arranged at equal annular spacingsrelative to one another around the full circumference of the housing.

In an embodiment of the invention, a continuous annular layer mixer isutilized as a premixing stage to create a fine dispersion of an uncuredsilicone polymer in a volume of filler. This material can then undergo aphase transformation under compressive, elongational and shear flowfields in an extruder to a compounded state, where dry filler is in aminority phase. First stage densification of the filler results inshorter incorporation times in the extruder and consequently significantimprovement in productivity.

The inorganic filler that can be used in the invention can be anyinorganic filler used in blends with silicone polymers. Examples ofinorganic fillers include a reinforcing silica such as fumed silica orprecipitated silica or a silica that has been surface-treated with anorganosilicon compound such as an organopolysiloxane,organoalkoxysilane, organochlorosilane or a hexaorganodisilazane. Thefiller can be diatomaceous earth, finely crushed quartz, aluminum oxide,titanium oxide, iron oxide, cerium oxide, cerium hydroxide, magnesiumoxide, zinc oxide, calcium carbonate, zirconium silicate, carbon blackor ultramarine. A single filler or a combination of fillers can be usedto reinforce the silicone polymer.

The amount of the filler can be in the range of from about 5 to about200 parts by weight, desirably from about 10 to about 100 parts byweight and preferably from about 20 to about 60 parts by weight, per 100parts by weight of silicone polymer.

Residual silanol groups on the surface of a filler can govern strengthof hydrogen bonds between the silica and hydroxyl or oxygen groups inthe silicone polymer chain. High concentrations of residual silanols ina filler cause “structuring” or “crepe hardening” of the final productin storage. This effect leads to difficulties in the processing of thematerial after it has been stored for extended periods. If theconcentration of silanol functional groups in a filler is too high, atreating agent can be added to reduce the groups to a requiredconcentration. The silanol reactant treating agent can react to reduceavailable groups to a concentration of between about 8 to about 2hydroxyl groups/ (nanometer)² of filler, preferably between about 7 toabout 3 hydroxyl groups/(nanometer)² of filler. The surface-treatedsilica is a preferred filler in the invention, in an amount from about10 to about 100 parts by weight, preferably from about 20 to about 60parts by weight, per 100 parts by weight of silicone polymer.

The treating agent can be mixed into the filler to reduce filler silanolgroups, to improve dispensability of the filler and/or to reduce thetime required for aging of the silicone rubber, to prevent crepehardening and/or to regulate plasticity. The treating agent can be anorganosilane, a low-viscosity polysiloxane or a silicone resin, whichhas a silanol group and/or an alkoxy group having 1 to 6 carbon atoms.Examples include diphenyl-silanediol, dimethylsilanediol,methyltriethoxysilane and phenyltrimethoxysilane. The low-viscositypolysiloxane may contain one or more kinds of organic groups selectedfrom a methyl group, a phenyl group, a vinyl group and a3,3,3-trifluoropropyl group. The viscosity of the polysiloxane measuredat 25° C. is in the range of from about 1 to about 300 cP, preferablyfrom about 5 to about 100 cP. The treating agent can be added in anamount of from 0.1 to 100 parts by weight, desirably from 0.5 to about50 parts by weight and preferably from about 1.0 to about 20 parts byweight per 100 parts by weight of the filler. Preferred silanol-reactanttreating agents include silanol-stopped polydimethylsiloxane,octamethylcyclotetrasiloxane (D4) and hexamethyldisilazane (HMDZ).

The silicone polymer used in the composition of the present inventioncan be represented by recurring units of Formula I:

wherein, R¹ independently at each occurrence represents C₁₋₄ alkyl, orC₂₋₄ alkylene; R² independently at each occurrence represents C₁₋₄alkyl, C₁-C₄ haloalkyl or C₂₋₄ alkylene; R³ independently at eachoccurrence represents H, C₁₋₁₀ alkyl, C₂₋₄ alkylene, C₄₋₆ cycloalkyl, OHor C₁-C₄ haloalkyl; and n represents an integer from 1,000 to 20,000.

A further preferred composition comprises a silicone polymer wherein, R¹independently at each occurrence represents, CH₃ or CH=CH₂; R²independently at each occurrence represents, CH₃, CH=CH₂ or CH₂CH₂CF₃;R³ independently at each occurrence represents CH₃, CH=CH₂, OH orCH₂CH₂CF₃; and n represents an integer from about 4,000 to about 10,000.

Another embodiment provides a composition wherein the vinyl content ofthe silicone polymer ranges from about 0.05% to about 0.5% by weight ofthe silicone polymer.

The heat-vulcanizable silicone composition can also include otheradditives such as heat-resistance improvers such as oxides, hydroxidesand fatty acid salts of metals, vulcanization reverse inhibitors, flameretardants such as platinum compounds, discoloration preventive agents,plasticizers such as silicone oil, internal release agent such as metalsoaps, pigments and dyes.

Features of the invention will become apparent from the followingdrawings and detailed discussion, which by way of example withoutlimitation describe embodiments of the present invention.

FIG. 1 schematically represents a process according to the presentinvention.

In FIG. 1, the apparatus 10 of the invention includes a high speedmixing stage 12 and an extruder stage 14. High speed mixing stage 12 canrepresent a continuous annular layer mixer and can represent a singlemixer or a plurality of mixers arranged to operate in sequence. Theextruder stage 14 can be one or more of a co-rotating intermeshingdouble screw extruder, a counter-rotating double screw extruder or asingle screw extruder. Preferably, the extruder stage 14 is aco-rotating intermeshing double screw extruder or a single reciprocatingscrew extruder. When the extruder stage includes a plurality ofextruders, they can be connected sequentially or in tandem.

In the process of the invention, filler is contained in loss-in-weightfeeder 16 and is fed 18 along with silicone polymer 20 and treatingagent 22, into mixing stage 12.

In the mixing stage 12, the polymer, filler and agent are subjected to ahigh speed, high intensity force to produce a free flowing powder premix24. Adequate tip speed and residence time are required to break down thematerial and to coat the filler with polymer and wet the materials withtreating agent. The materials can be mixed at an element tip speed ofbetween about 3 m/s and about 100 m/s to form the premix. Desirably, theelement tip speed is between about 10 m/s and about 80 m/s andpreferably between about 15 m/s and about 60 m/s. Residence is the timerequired for material to pass through the mixer. Residence time can bebetween about 3 seconds (s) to about 5 minutes (min) when a single mixeris used. The residence time for a single mixer can be from about 5 s toabout 1 min and preferably can be between about 20 s to about 45 s. Themixing stag 12 can comprise a plurality of mixers. When two mixers areused in sequence, residence time for the two can be from about 5 s toabout 10 min or the residence time can be from a 10 s to about 5 min orpreferably 20 s to about 3 min. The mixing stage 12 can produce aproduct with a tap density of about 0.3 to about 0.6 or about 0.35 toabout 0.55 or even 0.36 to about 0.48.

Advantageously, the premix 24 can be used in a continuous process (asillustrated herein) or can be stored for example by being discharged toa storage area and distributed for later use. In FIG. 1, the premix 24is fed to extruder stage 14 where it is compounded with additives 26 anddevolatilized to produce a heat-vulcanizable silicone polymercomposition 30. Annular mixer 12 is connected in sequence withcompounding extruder 14 and can be adjacent and connect to extruder 14or can abut and be connected to the extruder 14 and the composition canbe devolatilized 28 in extruder 14.

FIG. 2 illustrates another embodiment of the invention. The apparatus 50of FIG. 2 includes continuous annular layer high speed mixer 52 and asecond mixer 54 arranged in sequence. Two or more mixers can be utilizedto provide increased residence time to provide a more consistent premix.The FIG. 2 apparatus includes extruder 56 connected in sequence downstream of mixers 52, 54 and subsequent extruder 58.

In the process illustrated in FIG. 2, loss-in-weight feeder 60 metersfiller 62 into first mixer 52 to be mixed with polymer 64 and treatingagent 66. The product from mixer 52 is charged 68 into mixer 54 where afree flowing powder premix 70 is produced, which is charged to extruder56 where additional treating agent is added 72 for further fillertreatment. Additional polymer can be added (not shown) in this step tomake low durometer material. Processing aids and other additives areadded 74 in this step to produce a product 76 which is charged intoextruder 58 for devolitilization 78. Extruder 58 produces a heatvulcanization silicone polymer composition 80 for further use. Extruder58 can be a helically driven extruder mechanism. Extruder 58 produces aheat vulcanization silicone polymer composition 80 for further use.

FIGS. 3 to 8 show a continuous annular layer mixer and associatedelements.

FIGS. 4 and 5 are representations of processing elements that can beincluded in a mixer that can be used in the invention. FIG. 3 is a sideelevational view of a continuous annular layer mixer 102 showing aplacement of the elements of FIGS. 4 and 5.

In FIG. 3, mixer 102 comprises a cylindrical housing 104 with centrallongitudinal shaft 106. The housing 104 is sealed at transverse ends byend walls 108, 110. The shaft 106 projects through both ends of thehousing 104 and is sealed by end walls 108, 110. Material feed 112 isattached to an upper part of housing 104 to open substantiallytangentially into the interior of the housing 104 and discharge 114 isprovided at a lower end of the housing 104 substantially tangentially tothe interior of the housing 104 and opening out from the interior of thehousing 104.

Processing elements of various designs are provided on shaft 106 ofmixer 102. FIG. 4 shows a mixing element 116 that extends from rotatableshaft 106 and projects radially from shaft 106 within the continuousannular layer mixer 102 of FIG. 3. Mixing elements 116 are axiallyaligned at 90° intervals as viewed along the shaft 106 longitudinal axisand as shown in FIGS. 7 and 8. Mixing element 116 includes stem 118extending from base 120 to terminate in distal fan-shaped head 122. Themixing element 116 is shown extending perpendicular to the base 120 withan angled paddle head 122. The element 116 is fixed in the base 120 at ahead pitch angle to provide a relatively increased conveying(propulsive) function or backmixing function, as the case may be.Various pitches of elements and descriptions of function are providedhereinafter with reference to FIGS. 6 to 9.

FIG. 5 shows cutting element 124 extending from base 126 and directedradially from the shaft 106 within the mixer 102. The cutting element124 includes stem 128 that flares outwardly 130 and bevels inwardly 132to form a cutting edge 134 at its distal head 136. The element 124 isshown extending perpendicular. The element 124 can be fixed at a cuttingedge pitch angle so as to control conveying and backmixing functions, asdescribed with reference to FIG. 6 and FIG. 9. The elements 124 areaxially aligned at 90° intervals as viewed along the shaft 106longitudinal axis and as shown in FIG. 8. FIG. 8 is a schematicrepresentation of a view along the shaft longitudinal axis 106 of acontinuous annular layer mixer, showing mixing elements 118 and cuttingelements 124 projecting from shaft 106.

FIG. 6 is a schematic representation of elements illustrating elementpitch with respect to the mixer shaft 106. FIG. 6 shows a first draw-insection (first section) 138 of mixer 106, wherein mixing elements 140are provided that are set with heads in an axial conveying direction142. Filler/treating agent/silicone polymer material is charged into themixer 106 via the feed 112 shown in FIG. 3 at first section 138 and isaccelerated and set in motion in an axial conveying direction 142 by themixing elements 140. The FIG. 6 illustrates pitch of element heads 122and 136 from an axis defined by the longitudinal axis 150 of the mixershaft 106 where degree of pitch is defined according to FIG. 9 showing acompass 146 with abscissa 148 and ordinate 152. Elements pitched at anangle between 90° and less than 180° impart a conveying function, whileelements pitched at an angle between 0° and less than 90° impart aholding function. The mixing elements 140 extend close to the inner wallof housing 104 to avoid dead space and, as illustrated with reference toFIG. 7 elements 116, the mixing elements 140 are spaced around thecircumference of shaft 106 at about 90° intervals.

The mixing elements 140 are set into a conveying direction at an angleof about 138° from a perpendicular defined by the compass 146 of FIG. 9,wherein abscissa 148 coincides with mixer shaft longitudinal axis 150and ordinate 152 is perpendicular to the axis 150. Rotation of conveyingelement 140 creates a centrifugal force which flings material into theform of a ring at a radial outer end of element 140. Element 140 pitchthen causes the charged material in the form of the ring to advancehelically through the mixer 102 interior.

A second section 154 of the mixer 102 includes cutting elements 156 thatare pitched at a conveying direction angle of about 118° C. fromperpendicular. The cutting elements 156 are circumferentially spaced atabout 90° around shaft 106 as shown along with conveying elements 140.This spacing is illustrated in FIG. 9. The cutting elements 156 extendclose to inner wall of the housing 104 to avoid dead space. The elements156 act to separate clusters of material to accelerate wetting offiller.

A third section 158 comprises rearward pitched conveying elements 160 toprovide backmixing and increased residence time.

In an embodiment of the invention, the continuous annular layer mixer102 has a sequence of at least a first section comprising mixingelements, a second section comprising cutting elements and a thirdsection comprising mixing elements. The sections can include otherelements besides the specified mixing or cutting elements. For example,the second section can comprise cutting and mixing elements. The firstsection can comprise forward pitched elements, the second section cancomprise forward and neutral elements and the third section can compriserearward pitched elements for increased residence time. The totalelements of the continuous annular layer mixer 102 can comprise about 5to about 80% first section elements, about 10 to about 85% secondsection elements and about 0 to about 75% third section elements;desirably about 10 to about 65% first section elements, about 10 toabout 65% second section elements and about 10 to about 75% thirdsection elements; or preferably about 15 to about 55% first sectionelements, about 10 to about 45% second section elements and about 20 toabout 65% third section elements.

FIG. 6 illustrates an embodiment of the invention, wherein thecontinuous annular layer mixer 102 has a sequence of a first section 138comprising forward pitched mixing elements 140, a second section 154comprising forward pitched cutting elements 156 and a third section 158comprising rearward pitched mixing elements 160. As shown in FIG. 6, themixer 102 can terminate in a fourth section 162 comprising a forwardpitched cutting element 164 followed by a neutral element 168 forejecting the premix. The second section 154 can also include a forwardpitched mixing element 166.

According to the invention, premixing in the continuous annular layermixer creates a fine dispersion of an uncured polymer in a volume offiller. This material then undergoes a phase transformation undercompressive, elongational and shear flow fields in an extruder to acompounded state where the dry filler is a minority phase. Densificationof the filler in the annular layer mixer results in shorterincorporation times and consequently significant improvement inproductivity.

These and other features will become apparent from the followingdetailed discussion, which by way of example without limitationdescribes preferred embodiments of the present invention. In theExamples, premix quality is characterized by tap density, BET surfacearea, solution and dry powder particle size. The premix material isexamined by scanning electron microscopy, transmission electronmicroscopy and compression testing.

EXAMPLE 1

A Drais KTT continuous annular layer mixer is provided with the elementconfiguration described in Table 1.

TABLE 1 Element No./ Description Angle* Description 1 139 forwardconveying and mixing element 2 136 forward conveying and mixing element3 139 forward conveying and mixing element 4 135 forward conveying andmixing element 5 119 forward conveying and cutting element 6 117 forwardconveying and cutting element 7 122 forward conveying and mixing element8 62 rearward mixing element 9 68 rearward mixing element 10 71 rearwardmixing element 11 70 rearward mixing element 12 70 rearward mixingelement 13 69 rearward mixing element 14 69 rearward mixing element 1567 rearward mixing element 16 67 rearward mixing element 17 124 forwardconveying and cutting element 18 185 neutral cutting element *angle indegrees from perpendicular as defined by the compass 146 of FIG. 9

A silicone gum is charged into the mixer by means of a Doering pump(p=240 psi) at a rate of 40 lbs/hr and pretreated fumed silica ischarged by means of a loss in weight Acrisson feeder at a rate of 60lbs/hr. Both feeds are at room temperature. The mixer is operated at3000 rpm at an amperage of Discharge temperature increases from 81° F.to 89° F. and shell is consistent at about 73° F. Seven samples areprepared with a tap density of between 0.40 to 0.42.

EXAMPLE 2

The Drais mixing step of Example 1 is repeated with 63 parts of fumedsilica and 100 parts of a polymer gum. The product from the Drais mixingstep is immediately charged into a Banbury mixer where it is compoundedwith 2.5 parts methoxy stopped fluid treating agent 2.5 parts of silanolfluid treating agent/processing aid and 0.8 parts of vinyl methoxysilane crosslinker and then cured with 1.2 parts catalyst for 10 minutesat 350° F. and post baked at 450° F. Physical properties obtained for a75 Durometer product are provided in Table 2.

EXAMPLE 3

As a comparison, filler and polymer are added directly to the Banburymixer and are compounded with the same materials and cured in the samemanner as in Example 2. Physical properties obtained for a 75 Durometerproduct are provided in Table 2.

TABLE 2 Property/Example 2 3 Shore A Hardness 72 76 Elongation 395 366100% Modulus 322 355 Tensile 1277 1302 Tear B 148 153 Specifice Gravity1.226 1.203

EXAMPLE 4

The Drais mixing step of Example 1 is repeated with 63 parts of fumedsilica and 100 parts of a polymer gum. The product from the Drais mixingstep is immediately charged into a Banbury mixer where it is compoundedwith 2.5 parts methoxy stopped fluid treating agent, 2.5 parts ofsilanol fluid treating agent/processing aid and 0.8 parts of vinylmethoxy silane crosslinker. Samples of the materials are compounded inthe Banbury mixer at various RPM's. The compounded samples are curedwith 1.5 parts of 2,4- dichlorobenzoyl peroxide for 12 minutes at 260°F. The resulting sheet samples are post baked at 200° C. for 4 hours.Physical properties for 75 Duorometer samples are provided in Table 3.

TABLE 3 Property/Example RPM's 1400 2000 2800 3200 Shore A Hardness 7677 75 75 Elongation 324 347 308 323 100% Modulus 410 407 385 366 Tensile1327 1398 1210 1220 Tear B 134 126 126 130 Specifice Gravity 1.207 1.2091.198 1199

EXAMPLE 5

The Drais mixing step of Example 1 is repeated with 63 parts of fumedsilica and 100 parts of a polymer gum. The product from the Drais mixingstep is continuously charged into a twin screw, co-rotating intermeshingextruder for compounding and then continuously into a single screwreciprocating extruder for homogenization and stripping. The finalproduct includes 1.35 parts vinyl diol crosslinker, 2.0 parts of silanolfluid treating agent/processing aid and 3 parts of a methylvinyl sourceas a crosslinker/plasticizer. The product has <1% volatiles. The productis cured with 1.5 parts of 2,4-dichlorobenzoyl peroxide for 17 minutesat 260° F. Product sheets are post baked at 200° C. for 4 hours.Physical properties for 75 Duorometer samples are provided in Table 4.

EXAMPLE 6

The Drais mixing step of Example 1 is repeated with 61 parts of fumedsilica and 100 parts of a polymer gum. The product from the Drais mixingstep is continuously charged into a twin screw, co-rotating intermeshingextruder for compounding and then continuously into a counter-rotating,non-intermeshing twin screw extruder for homogenization and stripping.The final product includes 1.0 parts of silanol fluid treatingagent/processing aid and 0.5 parts of a methylvinyl source as acrosslinker/plasticizer. The product has <1% volatiles. The product iscured with 1.5 parts of 2,4-dichlorobenzoyl peroxide for 17 minutes at260° F. Product sheets are post baked at 200° C. for 4 hours. Physicalproperties for 40 Durometer samples are provided in Table 4.

TABLE 4 Property/Example 5 6 Shore A Hardness 70.1 37.8 Elongation 327519 100% Modulus 420 108 Tensile 1467 1078 Tear B 123 69 SpecificeGravity 1.21 1.106

The results show that a premix can be continuously prepared in a highspeed mixing stage to form a free-flowing particulate concentrate thatcan be continuously charged to compounding apparatus to prepareheat-vulcanizable silicone compositions.

While preferred embodiments of the invention have been described, thepresent invention is capable of variation and modification and thereforeshould not be limited to the precise details of the Examples. Theinvention includes changes and alterations that fall within the purviewof the following claims.

What is claimed is:
 1. A process of making a filled heat-vulcanizablesilicone composition, comprising: continuously feeding filler andsilicone polymer to a high speed mixing stage comprising a continuousannular mixer to form a free-flowing particulate concentrate;continuously discharging said free-flowing particulate concentrate fromsaid mixing stage to a compounding apparatus.
 2. The process of claim 1,comprising continuously compounding said concentrate in said compoundingapparatus to form a filled heat-vulcanizable silicone composition. 3.The process of claim 1, wherein said compounding apparatus is ahelically driven extruder mechanism.
 4. The process of claim 1, whereinsaid mixing stage comprises two continuous annular mixers connected insequence, comprising: mixing said filler with said silicone polymer insaid continuous annular layer mixers for a residence time of betweenabout 5 s and about 10 min.
 5. The process of claim 1, wherein saidmixing stage comprises at least two continuous annular mixers connectedin sequence, comprising: mixing said filler with said silicone polymerin said continuous annular layer mixers for a residence time of betweenabout 10 s and about 5 min.
 6. The process of claim 1, wherein saidmixing stage comprises two continuous annular mixers connected insequence, comprising: mixing said filler with said silicone polymer insaid continuous annular layer mixers for a residence time of betweenabout 20 s.
 7. The process of claim 1, wherein said mixing stage is acontinuous annular layer mixer and said compounding apparatus is ahelical driven extruder mechanism.
 8. The process of claim 7, whereinsaid continuous annular layer mixer is adjacent and connected to saidcompounding apparatus.
 9. The process of claim 7, wherein saidcontinuous annular layer mixer abuts and connects to said compoundingapparatus.
 10. The process of claim 7, comprising: mixing said fillerwith said silicone polymer in said continuous annular layer mixer at anelement tip speed of between about 3 m/s and about 100 m/s to form apremix; and discharging said premix to said compounding apparatus. 11.The process of claim 7 comprising: mixing said filler with said siliconepolymer in said continuous annular layer mixer at an element tip speedof between about 10 m/s and about 80 m/s to form a premix; anddischarging said premix to said compounding apparatus.
 12. The processof claim 7, comprising: mixing said filler with said silicone polymer insaid continuous annular layer mixer at an element tip speed of betweenabout 15 m/s and about 60 m/s to form a premix; and discharging saidpremix to said compounding apparatus.
 13. The process of claim 7,comprising: mixing said filler with said silicone polymer in saidcontinuous annular layer mixer for a residence time of between about 3 sand about 5 min to form a premix; and discharging said premix to saidcompounding apparatus.
 14. The process of claim 7, comprising: mixingsaid filler with said silicone polymer in said continuous annular layermixer for a residence time of between about 5 s and about 1 min to forma premix; and discharging said premix to said compounding apparatus. 15.The process of claim 7, comprising: mixing said filler with saidsilicone polymer in said continuous annular layer mixer for a residencetime of between about 20 s and about 45 s to form a premix; anddischarging said premix to said compounding apparatus.
 16. The processof claim 7, wherein said continuous annular layer mixer has a sequenceof at least a first section comprising a mixing element, a secondsection comprising a cutting element and a third section comprising amixing element.
 17. The process of claim 1, wherein said continuousannular layer mixer has a sequence of at least a first sectioncomprising a forward pitched mixing element, a second section comprisinga forward pitched cutting element or neutral cutting element and a thirdsection comprising a rearward pitched mixing element.
 18. The process ofclaim 1, wherein said continuous annular layer mixer comprises about 5to about 80% first section elements, about 10 to about 85% secondsection elements and about 0 to about 75% third section elements. 19.The process of claim 1, wherein said continuous annular layer mixercomprises about 10 to about 65% first section elements, about 10 toabout 65% second section elements and about 10 to about 75% thirdsection elements.
 20. The process of claim 1, wherein said continuousannular layer mixer comprises about 15 to about 55% first sectionelements, about 10 to about 45% second section elements and about 20 toabout 65% third section elements.
 21. The process of claim 1, whereinsaid continuous annular layer mixer additionally comprises a finalsection comprising forward pitched cutting or forward pitched mixingelements.
 22. A process of making a filled heat-vulcanizable siliconecomposition, comprising: mixing a filler with a silicone polymer in acontinuous annular layer mixer at an element tip speed of between about3 m/s and about 100 m/s to form a premix; and discharging said premix toa next processing apparatus.
 23. The process of claim 22, wherein saidtip speed is between about 10 m/s and about 80 m/s.
 24. The process ofclaim 22, wherein said tip speed is between about 15 m/s and about 80m/s.